FIG. 5 is a cross-sectional view of a conventional DBR wavelength-tunable semiconductor laser. The laser includes an n-type indium phosphide (InP) substrate 2. In the structure, the substrate 2 also functions as a cladding layer confining light generated in an active layer to light guide layer. In the structure of FIG. 2, an n-type indium gallium arsenide phosphide (InGaAsP) light guide layer 3 is disposed on the cladding layer 2. The semiconductor laser of FIG. 5 includes three serially disposed sections along the common substrate 2 and the light guide layer 3. Those sections are an active section 8, a phase control section 9, and a wavelength tuning section 10. In the laser, an n-type InP barrier layer 4 is disposed on the light guide layer 3 within the active section 8. An InGaAsP active layer 5 is disposed on the barrier layer 4 within the active section 8. A p-type InP cladding layer 6 is disposed on the active layer 5 in the active section 8 and on the light guide layer 3 in the phase control and wavelength tuning sections 9 and 10. A p-type InGaAsP contacting layer 7 is disposed on the cladding layer 6 throughout the active, phase control, and wavelength tuning sections 8, 9, and 10. A common electrode 1 is disposed on the substrate 2. Electrodes 11, 12, and 13 are disposed on the contacting layer 7 in the active phase control, and wavelength tuning sections, respectively.
The laser of FIG. 5 includes opposed facets 15 between which the active, phase control, and wavelength tuning sections 8, 9, and 10 are serially disposed. A diffraction grating structure 16, i.e., a Bragg reflector, is present at the interface of the light guide layer 3 and the cladding layer 6 in the wavelength tuning section 10. In other words, the interface between the light guide layer 3 and the cladding layer 6 is not planar and includes periodic features that interact with light propagating through the light guide layer 3 in the manner of a diffraction grating. In operation, the semiconductor laser generates light in the active layer 5. The light travels in the light guide layer 3, oscillating between and emerging from the respective facets 15 as light beams 14 shown in FIG. 5.
In the operation of the semiconductor laser of FIG. 5, a positive voltage bias is applied to each of the electrodes 11, 12, and 13 with respect to the common electrode 1. By employing separate electrodes, the amount of current flowing within each of the sections of the laser transverse to the layers can be controlled and the wavelength of light produced by the laser can be tuned, i.e., controlled, over a range of wavelengths. When a forward bias voltage is applied across the electrodes 11 and 1, i.e., in the active section 8, holes and electrons are injected into the active layer 5, producing light. The light propagates from the active layer 5 to the light guide layer 3 which has a lower index of refraction than the active layer 5 and a higher index of refraction than the substrate 2 and the cladding layer 6 for confining the light within the light guide layer 3. The light oscillates within the light guide layer 3 between the facets 15. When sufficient current flows between the electrodes 11 and 1, i.e., when the current threshold of the laser is exceeded, laser oscillation occurs and the light beams 14 are monochromatic.
At the same time light is being produced at the active layer 5, the diffraction grating 16, i.e., the Bragg reflector, interacts with the light propagating within the light guide layer 3 and reflects a predetermined wavelength of that light with a higher efficiency than light of other wavelengths. The forward bias voltage applied between the electrodes 13 and 1 causes charge carriers to be injected into the Bragg reflector structure 16 in the wavelength tuning section and decreases the refractive index locally because of the plasma produced by the injected charge carriers. As a result, the wavelength that is reflected with highest efficiency by the Bragg reflector structure 16 is shifted toward shorter wavelengths with increasing forward bias. In other words, the wavelength of the light emitted by the laser of FIG. 5 can be altered by changing the voltage bias applied between the electrodes 13 and 1, i.e., in the wavelength tuning section.
In order to optimize the light output at the selected wavelength, the phases of the light propagating in the light guide layer 3 in the active section and in the wavelength tuning section are matched by controlling the current flowing between the electrodes 12 and 1 in the phase control section 9. The voltage bias applied across the electrodes 12 and 1 is determined by observing the light output of the laser and adjusting the voltage bias to maximize the light output.
Although most of the light output of the laser of FIG. 5 is produced in the active layer 5, because each of the phase control and wavelength tuning sections 9 and 10 includes pn junctions that are forward biased, additional light output occurs in each of those sections. This light output is detrimental and can interfere with the propagation of the light that is generated in the active layer 5. In addition, because the current flow in the forward biased junction in the wavelength tuning section is relatively high, considerable heat is generated in that section during operation of the laser. The resulting temperature rise limits the light output of the laser and/or contributes to premature failure of the laser.